1. Field of the Invention
The present invention relates to devices for the manufacture of finned heat transfer tubing, and more particularly to a device for winding spiral fins onto oval tubing from one or several continuous lengths of metal strip.
2. Description of the Prior Art
Finned tubing is used primarily in heat exchangers where a first carrier medium, normally a liquid, passes through the inside of the tubing, while a second carrier medium, normally air or a gas, flows transversely to the tubing, in contact with its fins, so that heat is transferred from one carrier medium to the other. It is known to manufacture such heat transfer tubing by winding onto a length of smooth tubing a continuous metal strip, whereby the metal strip is bent edgewise to form a helical fin around the tubing.
In order to improve the contact between these spiral fins and the tubing, it has already been suggested to wind the fins from a metal strip whose contact edge is angled off to provide a narrow supporting edge engaging the surface of the tubing. Obviously, the angled edge on the metal strip greatly effects its edgewise bending characteristics, by shifting the neutral axis of the strip profile much closer to the contact edge. One desirable result of this change is a reduction in the previously encountered buckling tendency of the metal strip on its contact edge under the bending compression. This advantage, however, is only obtained at the cost of greatly increased elongation at the outer edge of the strip.
It has further already been suggested to apply this manufacturing method to oval tubing. The latter is preferable to round tubing, inasmuch as a greater contact area for heat transfer to the gaseous carrier medium is obtained, within a given flow cross section for that carrier medium. A method and apparatus for obtaining such oval finned tubing is disclosed in the German Patent No. 1,402,779 and in the corresponding British Patent Specification No. 947,544. These prior art patents disclose a machine designed for automatically winding a metal strip onto an oval length of tubing, after the contact edge of the strip has been bent over at a 90.degree.-angle. In practice, however, it was found that this machine, as proposed, was not capable of trouble-free operation, due to the tendency of the angled contact edge to buckle under the bending compression, especially when the latter reaches its peak on the small radius of the oval tubing contour.
Other prior art suggestions using a fin strip profile with an angled contact edge feature a contact edge of such a width and thickness that the neutral axis during bending practically coincides with the angled edge itself, meaning that virtually no longitudinal contraction takes place, while the upstanding portion of the fin is elongated as a function of its radius from the tubing axis. Such a large angled contact edge effectively eliminates any buckling tendency at that edge during winding, by eliminating the bending contraction through a shift of the neutral axis into the angled edge itself.